To raise transfer efficiency, digital communication devices that transfer information by modulating a carrier wave using a digital information signal (baseband signal) have been developed in recent years.
In digital communication, frequencies can be more efficiently used by raising the transfer speed and converting single frequencies into multichannels for use by several users. Raising the transfer speed, however, leads to deterioration in quality due to fading.
A number of techniques to counteract this problem have been developed. A representative technique is the adaptive array method. This method adaptively generates a directivity pattern using a plurality of antennas and generates an electromagnetic wave so that it only reaches a specified user. First, consider an adaptive array apparatus that has four communication subsystems that each include a transmission circuit, a reception circuit, and an antenna. Separate directivity patterns for transmission and reception can be generated for each communication subsystem by adjusting the gain and phase of each transmission circuit during transmission and by adjusting the gain and phase of each reception circuit during reception. The adaptive array method is described in detail in Adaptive Signal Processing for Spatial Regions and Its Technical Applications (in “Transactions of the Institute of Electronics and Communication Engineers of Japan”) Vol. J75-B-II No. 11, November 1992.
To perform bidirectional communication using the adaptive array method, it is desirable to have directivity patterns formed by both devices in communication. When this is applied to mobile communication, however, the physical limitations on the size of the mobile devices and the number of antennas used by them make it effectively impossible for mobile devices to form a directivity pattern. Accordingly, the base station forms separate directivity patterns for both transmission and reception. This means that during transmission, the base station forms a directivity pattern that is the same as the ideal directivity pattern that was formed during reception and transmits signals.
The adaptive array method has a problem in that it has been difficult for the base station to form the same directivity pattern during reception and transmission. A directivity pattern is formed by adjusting the gain and phase for each antenna. However, even if transmission is performed with the same phase as was used during reception, differences exist in the propagation characteristics (in particular, phase shift characteristics) of the reception circuit and transmission circuit. These differences have prevented the formation of the same directivity patterns for reception and transmission. The differences in propagation characteristics between the transmission circuit and reception circuit are due to the differences in circuit construction. These differences would still be present even if the same circuit construction were used, due to inconsistencies in the circuit components. This is to say, inconsistencies in the characteristics of actual circuit components lead to inconsistencies in radio circuits that are formed using such components.